1. Technical Field
The present invention relates to an document reading apparatus and an document reading method. In addition, the invention further relates to a program for causing an document reading apparatus to execute the steps of an document reading method.
2. Related Art
In the technical field to which the present invention pertains, a device that controls the rotation speed of a motor on the basis of the pulse cycle of a pulse signal that is outputted from a rotary encoder is known. When such a controller of the related art controls the rotation speed of a motor, a speed that is calculated on the basis of the last-acquired pulse time interval is taken as the current rotation speed of the motor. An example of such a control of the related art is described in JP-A-2001-251878.
That is, in such a related-art control, the speed that is used for controlling the rotation speed of the motor is calculated on the basis of the last-acquired pulse time interval. If the speed that is calculated on the basis of the last-acquired pulse time interval is simply regarded as the current rotation speed of the motor, and under the condition that the actual rotation speed of the motor is getting lower and lower, it follows that the actual rotation speed of the motor is slower than the speed calculated on the basis of the last-acquired pulse time interval.
Under such a condition, it further follows that a speed that is higher than the actual rotation speed of the motor is used for the computation of the controlling amount thereof when performing PID control on the rotation of the motor on the basis of the rotation speed thereof. Accordingly, under the condition that the actual rotation speed of the motor is decreasing, a difference between the target speed thereof and the current speed thereof will become smaller than otherwise. This means that the motor is controlled on the basis of a poorly calculated control amount that is smaller than a required controlling amount that should be calculated on the basis of the actual rotation speed of the motor. As a consequence thereof, if such a typical motor-controlling method of the related art is adopted, it is practically impossible or at best difficult to obtain a required controlling amount under the speed-decreasing condition explained above, which could result in any unintended stop of motor rotation.